Use a Hyphen When You Need One

The rule about hyphenating compound words is that the hyphen tends to go away if the word is common enough. We used to write “to-day” instead of “today,” for example. A more recent change is “web site” to “website,” now unhyphenated even when used as a compound adjective. 

Here’s one that should definitely still get the hyphen:

On April 30, the Pu‘u ‘O‘o crater on Hawaii’s Kilauea volcano, suddenly collapsed. It was the starting point for the volcano’s monthslong eruption, which went on to produce 320,000 Olympic-sized swimming pools’ worth of lava that transformed the landscape and ultimately destroyed 700 homes.

https://www.nytimes.com/2018/12/12/science/kilauea-hawaii-volcano-eruption.html

That’s a months-long eruption. That “nthsl” is just too long, and besides, where does the “s” go? I’m not aware of anything called a “slong.” It’s a good thing that the sentence didn’t have “swimmingpoolsworth.”

Remember, the goal of expository writing is to be clear. Try not to have bumps in your readers’ road.

PS—Those apostrophes in P’u ‘O’o aren’t contractions. They represent glottal stops, which English uses, but doesn’t have a letter for.